Lectins are proteins which are defined by their ability to bind carbohydrates specifically and to agglutinate cells. Lectins have been shown to be involved in a wide variety of cellular functions including cell-cell and cell-matrix interactions. Lectins are widespread among plants, invertebrates and mammals.
Animal lectins have been grouped into four distinct families: 1) C-type lectins, which include selecting; 2) P-type lectins; 3) galectins (formerly termed S-type lectins or S-Lac lectins); and 4) pentraxins Barondes S. H. et al. (1994) J. Biol. Chem. 269:20807-10!. The C-type lectins bind carbohydrate ligands in a Ca.sup.2+ -dependent manner and are structurally related to the asialoglycoprotein receptor. Selectins, a subcategory of the C-type lectins, are composite transmembrane molecules which are involved in cell-cell interactions. The selectins include lymphocyte homing receptors and platelet/endothelial cell surface receptors Stoolman (1989) Cell 56:907-10!.
Galectins bind specifically to .beta.-galactoside residues in a thiol-dependent manner. To date eight galectin types have been identified (galectin-1 through galectin-8). In addition to their affinity for .beta.-galactoside sugars, members of the galectin family share significant sequence similarity in the carbohydrate recognition domain (CRD; also referred to as the carbohydrate-binding domain). In most mammalian galectins, the CRD is encoded by 3 exons aid the majority of the residues conserved between galectins are encoded by the middle one of these three exons. Typically this region contains four .beta.-strands and intervening loops (these .beta.-strands are termed S3, S4, S5, and S6a/S6b according to Barondes S. H. et al., supra). Conserved residues or motifs located within the CRDs of mammalian galectins include H.sub.45, N.sub.47, P.sub.48, R.sub.49, V.sub.56, N.sub.58, W.sub.65, E.sub.68, F.sub.76 and G.sub.79 (numbering according to amino acid sequence of galectin-2 as shown in Barondes S. H. et al., supra). The importance of some of these conserved residues for binding carbohydrates has been confirmed by site-directed mutagenesis Abbott and Feizi (1991) J. Biol. Chem. 266:5552-57 and Hirabayashi and Kasai (1991) J. Biol. Chem. 266:23648-53!. All mammalian galectins which have been analyzed in detail recognize the same structural determinant on lactose (primarily the galactose residue with significant interaction with the glucose residue) and related .beta.-galactosides. In addition to binding .beta.-galactoside sugars, galectins possess hemagglutination activity.
Despite the fact that the cellular milieu contains a large number of .beta.-galactoside-containing glycoconjugates, few naturally occurring glycoconjugates have been shown to bind to specific galectins in vitro, suggesting that interactions between galectins and glycoconjugates are physiologically significant (i.e., galectins do not bind promiscuously to all naturally-occurring glycoconjugates). Laminin, a naturally occurring glycoprotein containing numerous polylactosamine chains, has been shown to be a natural ligand for galectin-1; galectin-3 has also been shown to bind laminin. Laminin is a component of the basal laminae, the extracellular matrix which underlies all epithelia and surrounds individual muscle, fat and Schwann cells. Interaction between cells and the basal laminae is known to influence the migration and/or differentiation of various cell types during mammalian development. Galectin-1 (also known as galaptin, L-14,L-14-I and BHL) is also known to bind to the polylactosamine-rich lysosome-associated membrane proteins (LAMPs) which can be found on the cell surface Do K. Y. et al. (1991) Biochem. Biophys. Res. Commun. 173:1123-28 and Skrincosky D. M. et al. (1993 Cancer Res. 53:2667-75; Erratum (1993) Cancer Res. 53:3652!, integrin .alpha..sub.7 .beta..sub.1 present on skeletal muscle Gu M. J. et al. (1994) J. Cell. Sci 107:175-81! and a lactosamine-containing glycolipid present on olfactory neurons Mahanthappa N. K. et al. (1994) Development (Camb.) 120:1373-84!. Galectin-3 (also known as Mac-2, EPB, CBP-35, CBP-30 and L-29) has been shown to interact with immunoglobulin E (IgE) and the IgE receptor Liu F. T. (1993) Immunol. Today 14:486-90 and Frigeri L. G. et al. (1993) Biochem. 32:7644-49, respectively!.
Structure of Galectins
Galectin-1 and -2 exist as homodimers composed of subunits of approximately 130 amino acids (.about.14 kDa); each subunit forms one compact globular domain which possesses carbohydrate binding activity (i.e., the CRD) Barondes S. H. et al. (1994), supra!. Galectin-3 has one CRD, a short N-terminal domain and an intervening proline, glycine and tyrosine-rich domain which consists of repeats of 7-10 conserved amino acids. These tandem repeats are characteristic of the collagen gene superfamily. The number of repeats varies between mammalian galectin-3 molecules and accounts for the differences in size between galectin-3 from different species of mammals. The N-terminal half of galectin-3 permits the molecule to undergo multimerization upon binding to surfaces containing glycoconjugate ligands. Galectin-4 exists as a monomer of about 36 kDa and has two CRDs connected by a link region that is homologous to the repeating domain of galectin-3 Oda Y. et al. (1993) J. Biol. Chem. 268:5929-39. Human galectin-7 exists as a monomer of approximately 14 kDa Magnaldo T. et al. (1995) Develop. Biol. 168:259-71 and Madsen P. et al. (1995) J. Biol. Chem. 270:5823-29!. Rat galectin-8 is a monomer of approximately 35 kDa and like galectin-4, has two CRDs in the same polypeptide chain joined by a short (.about.30 amino acids) link region Hadari Y. R. et al. (1995) J. Biol. Chem. 270:3447-53!. The link region of rat galectin-8 is not similar to either the link region of galectin-4 or to the proline, glycine and tyrosine-rich repeat domain of galectin-3.
Nomenclature For Galectins
Galectins have been discovered in a variety of mammalian species and in a variety of contexts; this led to the use of multiple names for the same galectin. Recently a consensus on nomenclature for galectins was reached Barondes S. H. et al. (1994), supra!. Individual mammalian galectins are now named by consecutive numbering (e.g., galectin-1). The genes encoding mammalian galectins are given the designation LGALS (lectin, galactoside-binding, soluble) followed by the number used to identify the protein (e.g., LGALS1).
Expression Of Mammalian Galectins
Galectins are expressed in a wide variety of tissues in mammals. The distribution of the different galectins is distinct but overlapping. Galectin-1 is expressed abundantly in muscle (skeletal, smooth and cardiac), neurons (both motor and sensory), thymus, kidney and placenta. Galectin-2 is expressed in hepatomas. Galectin-3 is expressed most highly in activated macrophages, basophils and mast cells; galectin-3 is also expressed in intestinal and kidney epithelial cells, vascular smooth muscle cells and in some sensory neurons. The expression of galectin-3 has been shown to be elevated in tumors Raz A. and Lotan R. (1987) Cancer Metastasis Rev. 6:433!. Rat galectin-4 is expressed in intestinal epithelium and the stomach (a galectin-4 homolog has been isolated from nematodes). Human galectin-7 is expressed in keratinocytes and its expression is markedly downregulated in SV40 transformed keratinocytes and in malignant keratinocyte cell lines Madsen P. et al. (1995), supra and Magnaldo T. et al. (1995), supra!. Rat galectin-8 is most highly expressed in lung with significant expression in liver, muscle (cardiac and skeletal) and spleen; low levels of galectin-8 are found in rat brain; expression of rat galectin-8 appears to be developmentally regulated as rat galectin-8 mRNA is expressed at very low levels in whole embryos while high levels of expression are found in adult rat tissues Hadari Y. R. et al. (1995), supra!.
Some galectins have been shown to be secreted (e.g., galectin-1, galectin-3 and galectin-7); however, all galectins characterized to date lack typical secretion signal peptides. There is direct evidence that some galectins are externalized by an atypical secretory mechanism. Galectins are not unique in being secreted by atypical secretory mechanisms; other secreted proteins such as interleukin-1, thymosin and fibroblast growth factor lack signal sequences.
Biological Activities Of Galectins
Galectins have been implicated in a wide variety of biological functions including cell adhesion, growth regulation, cell migration, neoplastic transformation and immune responses. Galectin-1 and -3 are the best characterized of the mammalian galectins. Galectin-1 is known to both promote and inhibit cell adhesion. In skeletal muscle, galectin-1 inhibits cell-matrix interaction and is thought to play a role in muscle development while in other cell types galectin-1 promotes cell-matrix adhesion. Galectin-1 has also been implicated in the regulation of cell proliferation and in some immune functions. Expression of galectin-1 has been shown to correlate with tumor metastasis potential Raz A. et al. (1986) Cancer Res. 46:3667-72!.
Like galectin-1, galectin-3 inhibits cell adhesion by binding to laminin. Galectin-3 plays a role in inflammation by binding to both IgE and the IgE receptor thereby causing activation of mast cells and basophils. Galectin-3 has been shown to concentrate in the nucleus of certain cell types during proliferation. Expression of galectin-3 is elevated in certain tumors, suggesting galectin-3 plays a role in metastasis. Indeed, overexpression of galectin-3 in a weakly metastatic cell line caused a significant increase in metastatic potential Raz A. et al. (1990) Int. J. Cancer 46:871-77!.
The expression of human galectin-7 appears to be limited to keratinocytes (Magnaldo T. et al., supra). Galectin-7 is thought to play a role in cell-matrix and cell-cell interactions as galectin-7 is found in areas of cell-cell contact (e.g., in the upper layers of human epidermis) and its expression is sharply downregulated in anchorage independent keratinocytes and is absent in a malignant keratinocyte cell line. Galectin-7 may be required for the maintenance of normal keratinocytes Madsen P. et al., supra!.
The expression of galectin-8 is developmentally regulated in the rat suggesting that galectin-8 regulates cell growth and embryogenesis. Rat galectin-8 is expressed in a wide variety of tissues in the adult including lung, liver, kidney, spleen and muscle (cardiac and skeletal) (Hadari Y. R. et al., supra).
Galectins represent an important family of proteins which are involved in regulating cell growth and development. Discovery of new molecules related to or in the mammalian galectin gene family is useful for the development of new diagnostic or therapeutic compositions.